The processes and plants used to manufacture Portland cement clinker are well known. Generally, the manufacturing process consists in preparing a raw meal (raw mixture) comprising a mixture of materials such as limestone (CaCO3), clay (argillaceous materials) (e.g., SiO2, Al2O3) and iron minerals (e.g., Fe2O3). Typically, the raw meal preparation includes the steps of drying, pulverizing and supplying said materials in adequate proportions to achieve a clinker with the required composition to obtain a raw meal mixture having the final quality desired. Once the raw meal is prepared, pulverized and homogenized with the required composition, the next steps in the process of manufacturing of the Portland cement clinker are carried out: i) feeding said prepared raw meal to a kiln passing through a pre-heater; ii) calcining said preheated raw meal to transform the CaCO3 into CaO and CO2; iii) feeding the calcined meal to a sintering kiln; and iv) sintering (also known as clinkering) the calcined meal to form the clinker phases such as tricalcium silicate (alite—C3S), dicalcium silicate (belite—C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) and other minor phases.
In order to carry out this process, high amounts of energy are required during the drying, calcining and sintering (clinkering) steps to maintain the high temperatures in the process, which being about 1450° C., whereby requires the consumption of large amounts of fuel.
Currently, the availability of fuels with a high sulfur (S) content and their lower cost represent an opportunity for the cement industry, however, their use requires special operation and handling with new methods to facilitate their use without detriment to the continuous operation of the kiln.
One of the available lower cost fuels is petroleum coke having a high sulfur content. Due to its nature, it typically has a high elemental sulfur content (S) ranging from 4.5% to more than 7%. The sulfur included in the fuel has a significant effect when included in the combustion process in the manufacture of cement clinker. On the one hand, this type of fuel generates the heat necessary to maintain the high process temperature. On the other hand, it produces SO2 which, added to the SO2 coming from the sulfur content in the raw materials conventionally used to form the raw meal, represents a significant amount. This sulfur content can produce blockages in the preheater when precipitating in the colder zones of the preheater or the kiln (800-900° C.) if steps or special measures are not taken or adequate compensating methods are not used for processing.
As is known, the SO2 contained both in the combustion gases as well as in the raw meals come in direct contact with the CaO (lime) and other compounds, forming sulfated calcium compounds (calcium sulfites and sulfates CaSO3 and CaSO4). When subjected to the high temperatures for forming the clinker phases, which are higher than the decomposition temperature, the sulfated compounds decompose again into SO2 and CaO. The latter reacts to form new clinker compounds, and the SO2 carried in the combustion gases returns back towards the kiln solids inlet to again react with incoming fresh CaO. In this way, a continuous cycle is set up with ever increasing sulfur concentrations. Upon reaching high enough concentrations of SO2, and having no outlet, sulfur compounds precipitate on the coldest areas of the preheater forming accumulations and blockages, such as in the preheater, the fume chamber, in the kiln itself, etc. This causes disturbances in the continuous operation of the process by reducing efficiency and, in more serious cases, stops the operation altogether; for example, when rings form in the clinkering kiln.
In the art, there have been efforts attempting to design installations, equipment and/or processes to enable the use of high sulfur content solid fuels by seeking to solve the problems related to the formation and excessive accumulation of SO2. However, the majority of the processes and/or plants to manufacture Portland cement clinker using high sulfur content solid fuel present certain disadvantages with respect to the complexity of the processes and equipment, as well as high costs. A more detailed discussion of efforts carried out in relation to this technology are established in U.S. Pat. No. 6,599,123 dated Jul. 29, 2003, entitled “Method for producing a cement clinker using coke with a high sulfur content”. One proposed solution to the problems associated with the use of high sulfur content solid fuel is that which is described in U.S. Pat. No. 6,383,283 granted on May 7, 2002 to Joseph E. Doument, entitled “Control of the production of cement clinker through the analysis of sulfur in the final product”. Unfortunately, the processes and/or plants referred to in said document, which use high sulfur content fuels in order to produce Portland cement clinker, are of no use for manufacturing a clinker at low temperatures where the raw meal does not include sulfurized components and agents that will encourage the creation of, new clinker phases.
Processes for the production of a cement clinker at low temperatures or with low energy consumption that reduce the consumption of fuel are done by introducing mineralizing agents into the raw feeding mixture. Conventionally, mineralizing agents such as fluorite (CaF2) and calcium sulfate (CaSO4) are used as components in raw meal. It is known that the addition of mineralizing agents may alter the speed at which the clinkering reaction occurs, thus speeding up the reaction and decreasing the energy required for the clinkering reaction.
For example, U.S. Pat. No. 5,698,027 describes a method and plant for producing mineralized Portland cement clinker, in which the preferred mineralizing source is a by-product of the desulferizing of combustion gases. The process and the plant described require equipment and additional stages for the treatment or feeding of the by-product of the combustion gases for the clinker production process. Furthermore, in order to avoid blockage or accumulation problems resulting from the decomposition of the calcium sulfate, the feeding of the mineralizing agent shall occur in: i) the calcination zone, ii) the tertiary air duct, or iii) the precalcination zone. U.S. Pat. No. 5,698,027 is wrong when it suggests that mineralizer must be fed directly into the raw meal introduced into the clinkering kiln. Likewise, it neither shows nor provides examples of how high sulfur content solid fuel (for example petroleum coke) can be used as a mineralizing source in all stages of the process.
Spanish Patent No. 8605210 granted to the Superior Council for Scientific Investigation, entitled “Procedure for obtaining low energy consumption clinker using fluorite and sulfates as raw components” provides a method in which raw meal is mixed together with fluorite and the sulfate component (CaF2+CaSO4). This Spanish patent does not mention the specific application of coke fuel with high sulfur content and its specific process, in which sulfur released by burning the fuel is fixed in the calcinated meal inside the clinkering kiln in order to create the sulfate component without the problems of accumulation and blockage that occur when using this kind of fuel. In due course, the method described uses a corrector of the resultant clinker composition in order to adjust the content of the phase formation agent.
International Publication No. WO 93/21122 dated Oct. 28, 1992, published in the name of Aalborg Portland A/S, entitled “Composition of Cement” describes the combination of a mineralized cement, in other a words a cement produced using a mineralizer, with an “expander” in order to obtain a significant reduction in the consumption of energy and low CO2 and NO emissions. It is known that adding a mineralizing agent to the clinker will increase its reactivity in order to use less in the preparation of cement. This document refers to the fact that the sulfate component may be introduced into the kiln as part of the fuel, in other words using fuel that contains sulfur. However, it does not describe the specific application of a fuel that contains sulfur nor of a solid coke fuel with high sulfur content or a specific process in which sulfur released by burning fuel is fixed in the calcinated meal in the clinkering kiln in order to create the sulfate component, without having to deal with the problems faced when using these types of fuel.
In accordance with the above, there is no document in this field that describes or suggests a process for producing Portland cement clinker at low temperatures fixing sulfur produced by burning coke fuel with a high sulfur content greater than 6.5% in which no sulfated compounds are used in the raw meal. Furthermore, none of the above-mentioned documents describe the phases of the clinker produced by means of this process. Therefore, a process is needed that will allow clinker to be produced more economically and more efficiently at low temperatures using high sulfur content fuels such as petroleum coke. This will avoid problems of CaSO4 decomposition as well as those problems linked to blockage and incrustations due to high concentrations of SO2 and/or SO3 in the system.
As a result, one of the aims of this invention is to provide a low energy process for producing cement clinker in which an increased percentage of sulfur generated by the combustion of solid fuel with a high sulfur content is established in the clinker, without the use of sulfated component in the raw meal and with the addition of an agent that encourages the formation of phases.
Another aim of this invention is to produce a clinker with new phases, but with characteristics similar to those of a Portland cement clinker.
A further aim of the invention is to provide a cement clinker that will eventually require the addition of a corrector of the resultant clinker composition depending on the quality and sulfur content of the solid fuel.